Field of the Invention
The present invention relates to a projection display apparatus that analyzes to pick up lights emitted from a plurality of reflective light valves, color-composes the analyzed lights, and projects the composite light (or the color-composed light) with a projecting optical system. In particular, the present invention relates to a method for preventing a ghost light from being produced in the projected light.
In the following, a conventional projection display apparatus will be described with reference to a diagrammatic illustration of the apparatus presented as FIG. 14.
Source light emitted from a light source 101 composed of a lamp and a concave mirror comprises a substantially parallel light flux of randomly polarized light. The source light is incident on a polarization converting device 102 so as to be converted into a S-polarized light in which the direction of oscillation of the electric vector (which will be hereinafter referred to as the polarization direction) is perpendicular to the plane of the drawing sheet. The light travels further and enters a cross dichroic mirror 103 in which a dichroic mirror 103B having a property of reflecting B (i.e. blue) light and a dichroic mirror 103RG having a property of reflecting R (i.e. red) and G (i.e. green) light are arranged orthogonal to each other to form an X-shape. The light incident on the dichroic mirror 103 is color-separated into a B light and a mixed light including R light and G light, which travel toward directions opposite to each other.
The B light thus color-separated is reflected by a deflecting mirror 104 to change its direction of travel and incident on a polarizing beam splitter 107B provided for the B light. On the other hand, the separated mixed light including R light and G light is reflected by a deflecting mirror 105 to change its direction of travel to enter a dichroic mirror 106 that has a property of reflecting G light, so as to be color-separated into G light that is reflected for further traveling and R light that is transmitted for further traveling. The R light and the G light thus separated are incident on a polarizing beam splitter 107R and a polarizing beam splitter 107G respectively provided for the respective colors.
Since All of the lights of the respective colors incident on the polarizing beam splitters for the respective colors are S-polarized, they are reflected by the polarizing splitting surfaces that are arranged substantially perpendicular to the plane of the drawing sheet as will be seen from FIG. 14 to travel further and enter respectively a reflective light valve 108B, a reflective light valve 108R and a reflective light valve 108G provided for the respective colors.
The lights of the respective colors incident on the light valves for the respective colors are subjected to modulation by color signals and reflected to be emitted from them. Then, they are incident on the polarizing beam splitter for the respective colors again, and the modulated lights (which are P-polarized lights) transmitted by the polarizing splitting surfaces are analyzed to be picked up. The non-modulated lights (which are S-polarized lights) are reflected by the polarizing splitting surfaces and thrown away in directions toward the light source.
Among the analyzed lights of the respective colors, the B light and the R light passes through half wave phase plates 109 and 110 respectively so as to be converted into S-polarized lights and enter a cross dichroic prism 111, which constitutes a color composing optical system, from different opposite incident surfaces. The analyzed G light is made incident on the dichroic prism with the P-polarized state unchanged.
The cross dichroic prism 111 that constitutes the color composing optical system is a composite prism in which a dichroic film 111R having a property of reflecting R light and a dichroic film B having a property of reflecting B light are arranged orthogonal to each other to form an X-shape. The B light incident on this prism 111 is reflected by the B light reflective dichroic film 111B, the R light incident on the prism 111 is reflected by the R light reflective dichroic film 111R, and the G light incident on the prism 111 is transmitted by the B light reflective dichroic film 111B and the R light reflective dichroic film 111R. Thus, color composing or color synthesis is attained, and the composite light is emergent from the cross dichroic prism 111.
The reason why the R and B lights are made S-polarized and the G light is made P-polarized before made incident on the cross dichroic prism 111 is to reduce loss of the light quantities of the incident lights that contribute to the composite light by utilizing optical characteristics of the dichroic films 111R and 111B so as to enhance brightness of projected images.
The RGB composite light emergent from the cross dichroic prism 111 passes through a quarter wave phase plate 112 so as to be converted into a circularly polarized light. The light is then made incident on a projection optical system 113 and projected onto a screen (which is not shown in the drawing).
The quarter wave phase plate 112 disposed between the cross dichroic prism 111 and the projection optical system 113 is provided for the purpose of preventing light reflected by lenses that constitute the projection optical system from being projected as a ghost light, as described in Japanese Patent Application Laid-Open No. 9-251150, which is owned by the assignee of the present patent application. Specifically, such a portion of a light to be projected incident on the projection optical system 113 that has been reflected by surfaces of a plurality of lens members that constitute the projection optical system is returned back to the quarter wave phase plate and transmitted by it. Thus, the R light component and the B light component in the returned light are converted from circularly polarized lights into P-polarized lights and the G light component is converted from a circularly polarized light into an S-polarized light. The light thus incident on the cross dichroic prism 111 is color-separated by that prism into the respective polarized lights and incident on the polarizing beam splitters for the respective colors, wherein the G light preserves its S-polarized state while the R light and the B light are converted into S-polarized lights by the half wave phase plates 110 and 109. The polarized lights incident on the polarizing beam splitters for the respective colors are reflected by the polarizing splitting surfaces and thrown away along the optical axes.
Reference is made here to Japanese Patent Application Laid-Open No. 2000-330196 as a patent document which discloses a projection display apparatus of the above-described type.
However, it was found that in the conventional projection display apparatus as described above, a ghost light is still generated even if a quarter wave phase plate for preventing projection of ghost lights is provided between a dichroic prism and a projection optical system.